How Can Half a Degree Ruin My Eggs?

Dr. Pia Sorensen of Harvard University tells us about her new book, Science and Cooking, and answers your questions about chemistry and physics in the kitchen.

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ERIC: The kitchen is a great place for science. Even when we're not actively experimenting with a recipe, we might be making observations, predictions, or even just wondering what it is that makes our favorite dish turn out just right.

Today on Pulsar, we'll explore the chemistry, physics, and biology of cooking and discover how everyday we're all scientists in the kitchen.

I'm your host, Eric. Thanks to Facebook Boston for supporting this episode of Pulsar.

My guest today is Dr. Pia Sorensen, a professor of chemical engineering at Harvard University and one of the authors of the new book "Science and Cooking". Dr. Sorensen, thank you so much for joining me on Pulsar.

PIA: Thank you for having me.

ERIC: I want to start out by asking you about your background, because when we host scientists at the museum who work at the intersection of two areas, they are often asked by our visitors, which one did you start out in.

So for you, did you start more on the science side or more on the cooking side?

PIA: I think it's one of those examples where I thought my background started in one corner, and then I look back and I realized it started even further back.

I'm a scientist. I, sort of, think of myself as a scientist, but going back, I grew up in Sweden in a family where a lot of the activity centered around food, slaughtering chickens at the family farm every fall, collecting all the berries, and all the apples, and all the vegetables in the garden and preserving them for the winter.

ERIC: So food was a big part of your life early on. Did that maybe lead to an interest in science?

PIA: Yeah, I think I was interested in food, and I think I was interested in the natural world around me and around us. And I think that's what ultimately led me to science. So as a PhD student, I studied, sort of, the molecular basis of the natural world around us.

There I did it in the context of cancer research. A lot of this same idea is there, it's this idea of trying to ask questions about what goes on around you, whether it's in the kitchen or, sort of, in nature, or really anywhere, anywhere else.

ERIC: And those questions, whether they're about birds in your neighborhood, or exploring other planets, or cooking, we can explore them all using the same basic principles of science.

PIA: Yeah, that's right, that's right. And I think really what fascinates me is that what most of us really do in our kitchens when we cook, or when we tried to perfect a recipe, or when we're just trying to figure something out, we really, sort of, think like scientists.

We're asking questions and we're trying to, sort of, devise experiments to try to test our hypotheses, to figure out what's the answer here, what's going on. And we're kind of doing that quite naturally. So what happens there is, I think, it's the scientists in us are coming out.

ERIC: I love that idea of thinking of something like perfecting a recipe as a science experiment, making a prediction, and testing it out.

PIA: Yeah, that's right, that's exactly it.

ERIC: So your new book that looks at science in the kitchen actually started as a class that you teach at Harvard University. So can you talk a little bit about the class itself and how you translated that into a book.

PIA: Yeah, so the class has been going for a while, since 2010. And it was started in collaboration with Ferran Adria and Jose Andres, so well-known chefs. And we've been teaching the class for the last 10, 11 years.

Every week we invite a famous chef, and we kind of really prompted them to talk about what they are excited about right now, what is their curiosity at the moment. And then, they cook for us. They show us things.

They talk about how they think about this as cooks. And then we use that as inspiration to ask questions about the science, understand the science.

Affiliated with this, we have Monday night public lectures, which I assume that some of the listeners to this podcast may have attended. They happened on Monday nights in the Science Center at Harvard. And we do the same thing.

The chef shows up, we talk about the science. And there's been a really wonderful community that has kind of developed over the years. And I think what we discovered is that there is a huge appetite for this, using that word intentionally.

And the idea of writing the book came out of it naturally. It made sense as we were approaching our 10 year anniversary that we should collect what I think are some of the most creative dishes, and do what we do in the class, like look at these recipes and, kind of, pull them apart and ask, what happens.

Why does this work? With this idea that the reason this is a delicious recipe and something you can serve in a restaurant is not a random thing. It's completely rooted in science. And as long as the science works, that dish is going to work.

ERIC: So we wanted to ask you some of the questions that we've received about cooking. And the first one was someone asking if everything in the kitchen can be broken down into physics and chemistry.

PIA: Well, I would add to that, I would add biology to that. Especially these days with the fermentation trend, so many of us are home fermenters these days.

So I think that the three of them really together kind of paint a whole picture. I, myself, come from a biology and chemistry background. So a physicist may not describe this the same way.

I think of it really as when we cook there are physical processes, and there are chemical processes, and the chemistry to me is kind of rooted in the molecules and how they're affected, and the physical processes involved, things like how does the heat transfer into a piece of food, and then causing this chemical process of the molecules being affected, the biological molecules like the proteins denaturing and causing something on a microscopic scale that we can see, that we can see on a macroscopic scale with the naked eye.

ERIC: And we've got a question from Scott about temperature. When things cook at 375 degrees Fahrenheit versus 400, they're both just really hot, so does it really make a difference?

PIA: That's a great question. I would say too that one of the things that is so interesting about temperature and cooking is that there are contexts where it doesn't seem to matter so much.

Like, we often start off our class baking cookies. And a lot of cookie recipes are quite robust, and the temperature really you can go plus or minus 10, 20 degrees, maybe even more. In fact, most of our ovens are not very well calibrated anyways.

So the temperature isn't what we think it is anyway when we put the cookies in the oven.

Some recipes are really quite robust. And then there are other recipes where the temperature, even half a degree, is going to completely mess up the dish. I think one example of that is sous vide eggs, the eggs that you cook at a certain temperature for a longer time.

And so when you take the egg out, it is exactly at 63 degrees, that's kind of the perfect egg. And if it is 62, you and I may not immediately notice, but a chef who does this all the time is going to notice, and may be quite upset that you cook the eggs wrong.

ERIC: Wow, that is a really narrow range. What's driving that sensitivity to temperature in some dishes?

PIA: Biological systems are often quite finely calibrated to work in very narrow temperature ranges. So we kill microbes at a certain temperature to make food edible.

At a lower temperature, those microbes would grow and be quite happy, and they would produce a beautiful sourdough loaf. But I think it really says something about the narrow temperature arrangement organisms and sort of biological materials what they're optimized for.

ERIC: Now, a lot of questions we received had to do with the texture of food. So can you talk about what's really happening when we see changes in texture while cooking.

PIA: There are so many ways of changing texture when we cook. One of them, for example, would be with egg whites, right? You take a liquid, you whip, whip, whip, whip, and it turns into this beautiful foam that kind of has a completely different texture in your mouth than the egg white does.

And in that case, it's a matter of incorporating air bubbles into the egg white, which kind of gives this airy consistency. You could imagine changing the texture by say, you cook a steak, and it goes from a soft steak to a firm steak.

And that tells us something about both that water evaporated, but also about the denaturation of the proteins, which formed a tighter network, which is stiffer.

So I think there's so much going on, on a texture level, but I would say that a lot of the changes we see can often be summarized by a few principles. So I think this idea that you can increase the density of a network, the way you do when you cook a steak, by having more denatured proteins, more cross links.

That is a common way. You do that when you cook jello, when you cook steak, to some degree even when you cook bread. And then there is the idea of increasing the density of particles.

So when you cook down a sauce, for example, and it goes from very liquid to quite dense, you're really just changing how dilute it is. So I think these two ideas, the increase in their perks and the packing of molecules are kind of two unifying factors that texture can be changed.

ERIC: And speaking of changes that we can observe while cooking, do you have any dishes that our listeners can try at home if they're interested in doing some experimenting in the kitchen?

PIA: I always encourage people to go and make sauerkraut. If they haven't done it, I think they should try it. They should look up the recipe online. The magic of it is that all it is cabbage, raw cabbage, and some salt.

You sprinkle the salt, it's like 2% salt, you sprinkle the salt over. You knead and knead until the water is pulled out of the cabbage through osmosis. Then, you just really tightly put it in a jar, and then you wait. And I think the magic of it is that it's so simple.

It's historically maybe one of the oldest culinary traditions we have. It's something many of us don't do on an ordinary basis. And what happens is all due to the microbes.

ERIC: Well, I wanted to finish by, sort of, looking at the big picture and asking why you think that cooking is a really great way to think about science.

PIA: I think, in general, what works well for so many of us with food and cooking is that we're so connected to it. And it tends to excite us. Good food excites us.

And things that make us excited, tend to be something that sparks our curiosity and makes us want to know more. So I think because of that, it's a great vehicle to kind of start thinking about what it goes on, and tap into to that curiosity a little bit.

ERIC: Well, Dr. Sorensen, I hope our listeners will think about all that next time they're making a meal. Thanks so much for talking with me today.

PIA: Thank you so much.

ERIC: "Science and Cooking: Physics Meets Food - from Homemade to Haute Cuisine" is now available wherever books are sold.

Until next time, keep asking questions.